For a successful pressing process of a dental ceramic it is important to perform the heating process based on an exactly defined heating profile. Such a profile with a first comparatively high initial heating rate which is followed by a lower second heating rate is apparent from for instance, DE 10 2008 012 578 B4 and corresponding U.S. Pat. No. 9,033,703, which is hereby incorporated by reference.
In principle, such a heating curve is favorable for a satisfactory press result because in this connection, before the pressing is started, that is to say at an uncritical point in time, strong heating is carried out, and during the pressing process itself the material is pressed carefully.
EP 1 915 972 B2 and corresponding U.S. Pat. No. 8,317,512, which is hereby incorporated by reference, discloses a further improvement in this regard according to which an integral is used across a temperature/time area for detecting the amount of heat introduced.
While the temperatures measured typically relate to the internal temperature of the furnace and in this respect not to the real temperature of a green product, the document WO 2014/131588 A1 discloses a method for detecting the temperature of the green product itself.
With relatively long green products, however, there is a certain temperature gradient within the green product, and using the above-mentioned new and basically favorable solution which is, however, not completely uncomplicated, the temperature of the green product can typically be measured at one position but not at every position.
A further problem is that with particularly configured temperature profiles the dead time of the system of muffle/green product delays the measured temperature profiles. It is strongly dependent on the mass of the green product and of the muffle. This means that particularly voluminous muffles and large dental restoration parts, such as fourteen-unit bridges, should be operated in accordance with even “sharper”, that is to say longer and more exact heating profiles.
In this respect, in order to comply with the necessary safety distance a holding time is typically inserted after the heating process which is to serve the temperature equalization and in which the hot external areas of the muffle give off their heat to the center of the muffle and thus to the green product.
However, this holding time prolongs the pressing cycle considerably depending on the size of the muffle, and in some cases this is not considered acceptable. Furthermore, depending on the muffle size used a corresponding control program comprising suitable temperature profiles and holding times must be selected. With today's large number of materials and programs for different muffles, the dental technician may make a wrong selection from time to time in the heat of the moment.